For some time, it has been known, that optical fibre in which the core is doped with particular substances, for example, rare-earth ions, has stimulated emission characteristics adapted to be used as laser sources and in optical amplifiers.
In fact, these fibres can be pumped with a light energy at a particular wavelength, that is capable of bringing the doped atoms to an excited emission, or metastable state in which state they remain for a relatively longer time.
When a fibre having a high number of atoms excited in the emission level is traversed by a light signal with a wavelength corresponding to such emission state, the light signal causes the stimulated transition of the excited atoms to a lower level and the light emission has the same wavelength, polarization and propagation direction as the input signal. Therefore, a fibre of this kind can be used to obtain optical signal amplification.
During the last several years, rare-earth doped optical fibre amplifiers, and more particularly, erbium-doped fibre amplifiers (EDFAs) have had a major impact in the field of lightwave communications. Their high gain, high power, and low noise has made them advantageous over electronic repeaters used in some optical fibre systems. As well their data modulation format, data bit rate, and insensitivity to input light polarization makes them a preferred choice for many optical systems.
However, one limitation of any rare-earth doped optical fibre amplifier is unequal gain over a range of frequencies or optical channels of interest, as well as for various input signal strengths (i.e. different saturation levels). Over a 35 nanometer gain bandwidth, erbium doped fibre amplifiers (EDFAs) typically exhibit a 10 to 15 dB small-signal gain variation. This variation becomes smaller when the amplifier operates in the saturated region; the gain variation almost completely vanishes and is typically smaller than 1 dB for a fully saturated amplifier. Therefore an ideal gain equalization circuit is preferably active in its characteristics, accommodating to the variable saturation level of the operating EDFA. In long chains of cascaded EDFAs small spectral gain variation can result in unacceptable large difference in received optical power and therefore, it is preferable to lessen even small spectral variation in gain.
To date, several gain equalization and flattening techniques have been proposed and described in a variety of prior art references. For example, gain clamping with enhanced inhomogeneous saturation is described by V. S. da Silva et al in Proc. OFC'93. paper THD2, P.174, 1993. One of the limitations of this method is the requirement that fibre be cooled to 77K. The use of passive internal/external filters has been explored by M. Tachibana, et al in IEEE Photonics Technol. Lett. 3, no. 2, 118, 1991, by M. Wilkinson et al. in Electron. Lett. 28, no. 2, p. 131, 1992, and by Kashyap et al in Electron. Lett. 29, no. 2, P. 154, 1993, and as well by Grasso et al, in Proc. OFC'91, paper FA3, p. 195, 1991. Another attempt to provide a doped optical fibre amplifier that is suitable for use over a range of frequencies is described in U.S. Pat. No. 5,245,467 entitled Amplifier with a Samarium-erbium Doped Active Fibre, issued Sep. 14, 1993 in the name of Grasso et al. Although the invention described in the patent works well at particular wavelengths and for particular signal strengths, it has been found to be limited at other wavelengths. However, the major limitation with most of these devices and methods is the requirement for bulk optics and non-standard components. The use of external active acousto-optic filters has been explored by S. F. Su et. al in IEEE Photonics Technol. Lett. 4, no. 3, p. 269, 1992; the drawback with this proposal is that it requires bulk optics, is complex in design, and has high loss.
One reference that attempts to overcome some of the problems inherent in rare-earth doped optical amplifiers is U.S. Pat. No. 5,050,949 entitled Multi-stage Optical Amplifier, in the name of DiGiovanni et al., issued Sep. 24, 1991. In this invention, a multi-stage optical fibre amplifier for providing gain equalization is disclosed. The amplifier comprises at least two stages of amplification where each stage comprises an amplifying fibre having a different gain spectrum. In one embodiment, the two stages, which can be pumped separately, have different dopant compositions to provide each stage with a different gain spectrum. Although DiGiovanni's solution may be adequate in some instances and for specific frequency ranges, equalization is limited to a relatively narrow bandwidth. Another proposal for amplification equalization is spatial hole burning in twin-core fibre proposed by R. I. Laming et al in Proc. OFC'93, paper ThD3, p. 175, 1993. However, one of the drawbacks of this solution is that it is relatively complex to design these fibres and as well, the use of non-standard fibre make this solution less attractive than others. Another solution to that attempts to provide gain equalization is the adjustment of input signal powers. Unfortunately, this scheme requires prior knowledge of the wavelength and strength of the input signal. As well, a precise adjustment of input power at each wavelength is required.
In general, all of the aforementioned references are somewhat limited in that they perform adequately for signals of particular wavelengths and intensities, but do not respond adequately in a dynamic sense by adapting to input signals having varying wavelengths and intensities of to provide a relatively spectrally flat output response. All of the aforementioned amplification techniques do not adapt attenuation to various signal power levels and amplifier saturation properties; and, most of these techniques require bulk, optical components.
It is an object of the invention to provide a rare-earth doped amplifier that has a relatively flat input/output response for various signal power levels and at various wavelengths.
It is a further object of the invention, to provide an optical fibre amplifier that can be fused to and is compatible with conventional optical fibres.